If I leave my MBP (mid 2010, newest model 15-inch) plugged in most of the time is there any battery capacity loss like older laptops did or has Apple managed to prevent this?_________________"You must control your future by taking command of your present, and fixing and learning from your past."

My 2007 MBP is almost always left plugged in. When I unplug it, it lasts maybe 3 hours doing video and audio. You should be good, assuming they didn't make it worse. I can't imagine they would have, though.

Avoid keeping the battery at full charge and high temperature. This is the case when placing a cell phone or spare battery in a hot car. Running a laptop computer on the mains has a similar temperature problem. While the battery is kept fully charged, the inside temperature during operation rises to 45°C (113°F).